The study of HCV infection, replication and development of new therapeutics has been hampered by the lack of a small animal model permissive to infection and replication. We have recently found a means to maintain human hepatocytes in mice for at least 5 months. We have demonstrated that these animals are susceptible to human hepatocytes in mice for at least 5 months We have demonstrated that these animals are susceptible to HBV/HDV infection/replication. We will now attempt to infect these mice with HCV. The goal of this proposal is two-fold. We plan to improve upon the human hepatocyte xenotransplant model by changing the transplantation strategies so that we can implant a larger number of cells. Moreover, we will characterize the implant by histological methods and gene expression profiles using microarray technologies to determine the similarity between the implanted cells and normal liver. We will then determine if this chimeric mouse is able to undergo HCV infection/replication. We will do this by implanting human hepatocytes isolated from HCV infected individuals, infection of normal hepatocytes ex vivo and in vivo with high titer HCV stocks, and infusion of an infectious RNA derived from a single clone. We will follow the animals for HCV infection by quantitating serum HCV RNA titers over time. Ultimately, we will combine the best transplant model with the best method for HCV infection to determine the highest possible HCV titer that can be achieved in mice. When we are successful, this animal model will be used for numerous studies. These include: gene therapy approaches to reduce or eliminate HCV infection, proliferation of quasi- species, viral pathogenicity, and the importance of the immune response in infection and pathogenicity. The development of a small animal model of HCV infection will be invaluable in our advancement for better knowledge and treatment of this devastating disease.